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New Detector Boosts Laser Communications

Why is it so tricky to deliver large amounts of data from space? One key issue is frequency — because the amount of data that can be transmitted varies with the square of the frequency, higher frequencies give you more bang for the buck. Moving the Deep Space Network from today’s X-Band (between 8.40 and 8.45 GHz for deep space work and between 8.45 and 8.50 GHz for near-Earth operations) to the Ka-Band (31.80 to 32.30 GHz) will increase the network’s capabilities by a factor of four or five.

But the real goal is optical communications, where the far narrower signal carries a vastly increased amount of information. We need that kind of data-packing not only to get around spectrum-crowding as more and more spacecraft need to talk, but also to handle the high resolution imagery and video we’ll want to see from future deep space missions.

“It can take hours with the existing wireless radio frequency technology to get useful scientific information back from Mars to Earth. But an optical link can do that thousands of times faster,” said Karl Berggren, assistant professor in the Department of Electrical Engineering and Computer Science at MIT.

So news from Berggren’s team at MIT is heartening. Researchers there have developed a new light detector that can use optical links to surmount far slower radio technologies. The detector works at the same wavelength used by the optical fibers carrying broadband signals to homes and offices, and may eventually lead to startling results including color video from the far corners of the Solar System.

And just as critical, we’ll be able to move more and more of the high resolution imagery from missions like Mars Global Surveyor back to Earth. These are incredibly bulky datasets, including the results from observations made with synthetic aperture radar, terrain-mapping radar, and hyper-spectral imaging, and they gobble up plenty of precious bandwidth.

We need detectors like this one because spacecraft are starved for power. Using superconductor technology and nanowires, the MIT design is incredibly sensitive — working down to the level of a single photon — meaning it can receive signals from smaller lasers. The design is also speedy and efficient at light-gathering.

Such detectors are only one step, but they are markers of our progress on the road to an interplanetary infrastructure of laser installations that far surpasses conventional radio links. And that leads, ultimately, to the kind of optical network that will receive laser signals from our first generation of true interstellar probes.

For more, see Rosfjord, Yang, Berggren et al., “Nanowire single-photon detector with an integrated optical cavity and anti-reflection coating,” in Optics Express Vol. 14, Issue 2 (23 January 2006), pp. 527-534. A PDF is available here.